On occasion, nearly all outboard motor owners have noted that their motors will suddenly rev up excessively without a corresponding increase in boat speed--and frequently with a noticeable drop off in boat momentum. This phenomenon is called cavitation. Cavitation is that condition that occurs when the propeller is brought into contact with excessively disturbed or turbulent water. Air is drawn from the water's surface through the propeller, which thus relieved of the burden of drawing heavier water into its slip stream, responds by revolving faster. Acting more proportionately on air than on water, the propeller's effectiveness is reduced; the blades merely churn instead of thrusting.
To prevent cavitation and improve performance, an outboard motor should be operated in such a position that the propeller's thrust is vertical to the water line and so that when the boat is planing or nearly planing the motor's anti-cavitation plate is parallel to the water's surface. All outboard motors are equipped with a means of angle adjustment. This may be accomplished by shifting a bolt location in a slotted member or by wing-nut adjusting bolts or by similar means dependent on the particular make of motor.
Boat manufacturers, too, have nearly universally established a 12 degree angle from perpendicular to the transom. This angle is provided for with a dual purpose: to permit free adjustment of the motor and also to prevent water turbulence and resulting propeller slipping.
On lightweight boats with relatively modest horsepower motors, it may be found that the angle of attack of the propeller determined by the motor angle of adjustment must be altered when boat load is increased or decreased. A motor which is mounted in such manner that its propeller is tilted downward and away from a vertical axis with the water line will cause a lightweight boat to squat and perhaps even fail to get up on plane. With a sufficiently large motor, which despite an incorrect angle of adjustment still succeeds in getting a boat up on plane, the steering characteristics will be difficult. The boat will tend to rear its bow high in the air and drop back onto the water again in a constant and annoying porpoising action. If, by contrast, the motor is kicked in too close to the transom, it will get up on plane readily but will then ride nose-heavy and plow the water. This will not only reduce speed but will tend to make the boat broach in turning and take on a snaking steering characteristic in straight-ahead operation.
The final proper engine angle at which a boat will handle at its best and offer maximum performance is the angle at which the propeller is driving parallel to both the boat's keel line and the water's surface.
The above background discussion is taken from pages 203-206 of The Encyclopedia of Outboard Motorboating by Hank Wieand Bowman (Library of Congress Catalog Card Number: 55-6649. Copyright 1955 by A. S. Barnes and Company, Inc., New York, N.Y.) which is incorporated by reference.
In the past, various devices have been employed with outboard and outboard-inboard motor boats providing so called power trim units for the boat to vary the planing attitude of the boat, and motor tilt devices to vary the tilt of the motor with respect to the boat transom. Such devices have included trim gauges of one type or another to indicate the degree of tilt of the outboard portion of the motor with respect to the transom. Such indicators may also indicate the distance that the motor is moved away from the transom which is simply another form of tilt indication with respect to the transom.
Such devices do not indicate the relative position of the outboard motor with respect to the water and give no accurate indication of whether the axis of the propeller shaft is parallel to the line of travel of the boat, or in other words whether the axis of the propeller shaft is parallel to the surface of the water. In such parallel condition, the motor should be perpendicular to,the surface of the water or line of travel of the boat.
It has long been a problem to present the maximum propulsion force in such boats both for economy purposes and high speed performances. Such problems are well typified by an upward direction of thrust of the propeller causing the well-known rooster tail with a consequent loss of energy and fuel and a downward thrust causing a bow-heavy boat.
For the foregoing reasons, there is a need for an economical device which can assist one in positioning the outboard motor with respect to the transom for maximum performance.